Air pressure actuated power clamps have been used for many years which employ straight piston rod stroke between opposed straight reaction guide tracks in which bearings for one end of parallel links are driven by the piston rod the other ends of which are pivotally connected to a clamp arm having a spaced pivotal connection to the clamp body. Actuation of the links toward a right angle relationship of link pivots to the reaction track articulates the clamp arm towards its clamping position. When the clamp arm is adjusted to provide maximum clamping pressure on a workpiece at standard factory air pressure such as 80 p.s.i. any travel to center or slight overcenter to a positive stop of the clamp arm has been found in most commercial clamps currently available to require a release pressure exceeding the 80 p.s.i. apply pressure by as much, for example, as 20 to 30 p.s.i. Accordingly, since this may result in a locked up clamp which cannot be released by standard air pressure such clamps are normally operated with a limited travel of the piston rod to a linkage angle short of 90.degree., e.g. in the order of 85.degree., to assure that supply line pressure will always release the clamp. Such practice, however, does not assure that clamping pressure will remain engaged in the absence of actuating air pressure even though a self-locking friction angle is attempted since vibration of the workpiece may permit the component of release force to gradually urge the linkage to a release condition. While it may be tolerable to leave air pressure applied under conditions where the workpiece and clamp remain stationary near a supply line, there are many requirements in industry where the workpiece travels on a pallet, truck or platform having air operated power clamps which must remain clamped while traversing substantial areas in the plant. For many years the only solution to this working condition has been to employ portable air pressure tanks mounted on the moving work platform thereby providing means for maintaining clamp actuating air pressure throughout required transport of the clamped workpiece.
Notwithstanding long recognized need for a locking power clamp to permit the use of portable clamps on moving workpieces without having an accompanying portable air supply, a satisfactory solution has proved to be extremely elusive. Attempts have been made to decrease static friction at the center or overcenter position through lubrication and low friction bearing materials such as Teflon without success. In one known commercial clamp the combination of Teflon bearings and a spring element to accommodate overcenter locking has provided initially acceptable release forces but unacceptable durability under life cycle tests leading to unacceptable higher release values as wear occurred in the Teflon bearings together with problems of spring breakage from fatigue.